The present invention relates to electrochromic devices, and in particular to electrochromic devices comprising an electrically conducting, electrochromic element and at least one layer of solidified electrolyte, and two or more electrodes for connection to an electric voltage supply. The invention also relates to addressing of an electrochemically active element.
Electrochromic materials exhibit colour changes or changes in optical density as a result of electrochemical reduction and/or oxidation reactions. An electrochromic material can either be present as a solid, or exist as molecular, neutral or ionic species in an electrolyte solution. These materials have been used for the creation of electrochromic cells, where the passage of electric charge causes colour changes in the materials. Electrochromic cells are used in electrochromic devices of different kinds, and two principal categories of these devices can be distinguished. The two-categories differ from each other mainly in the arrangement of the elements of the electrochromic cell.
The first category of electrochromic devices utilises a sandwich construction, and is used in applications such as automobile windows, building windows, sunglasses, large billboards, mirrors with variable reflectance, sunroofs etc. In this type of electrochromic device, continuous layers of electrochromic material and electrolyte (as well as other layers of e.g. ion reservoir material) are confined between two electrodes that completely cover the layers of electrochromic material and electrolyte. For the electrochromic device to be of use, at least one of said electrodes has to be transparent to let light through the device. This requirement is met in the prior art through the use of electrode materials such as indium-doped tin oxide (ITO), tin dioxide or fluorine-doped tin dioxide. The electrochromic-materials used in these applications vary, but are often based on heavy metal oxides such as WO3 or conducting polymers such as polyaniline or polypyrrole. The conducting, electrochromic polymer poly-(3,4-ethylendioxythiophene) (PEDOT) has attracted much study, and sandwich devices incorporating this polymer have been realised.
The second category of electrochromic devices aim at providing an electrically updateable display for realisation on a flexible support, U.S. Pat. No. 5,754,329 describes such a display, in which the electrodes of the electrochromic device are placed in one and the same plane, contacting a layer of electrochromic material for the generation of local colour effects at the interface between the electrochromic material and the electrodes. U.S. Pat. No. 5,877,888 represents a further development of this device, describing a two-sided display. However, the arrangement of the component layers of the electrochromic device is similar to that of the device of the U.S. Pat. No. 5,754,329 patent, considering that the electrodes on either side of the display support contact electrochromic material only, and the generation of electrochromic effects is confined to the area of the electrodes. The electrochromic materials that are used in these devices are described in detail in U.S. Pat. No. 5,812,300.
The electrochromic devices of the prior art have limitations in that any colour change effects are essentially confined to the area of electrodes. Prior art devices offer no versatility in the addressing of electrochromic material. As a result, the prior art suffers drawbacks as to the potential for generating innovative and versatile electrochromic devices. Furthermore, the materials used in electrochromic devices of the prior art suffer drawbacks as to environmental friendliness, processability and economy. Thus, there is a demand for electrochromic devices that improve the art and do not suffer the disadvantages of the prior art.
An object of the present invention is to meet this demand, by providing an electrochromic device that allows the electrochromic material to be addressed via the electrolyte, so that the electrode architecture is not limited by the requirement that the electrodes of the voltage supply be in direct electrical contact with the electrochromic material for electrochromic effects to occur. In embodiments of the invention, the electrochromic material used should exhibit colour change at locations distant from the immediate area of the electrodes, in response to an electric field within the electrolyte between the electrodes.
Another object of the present invention is to advance the art of electrochromic devices, by providing an electrochromic device, for example an electrochromic display, which utilises a combination of materials that are simple to use, compatible with a flexible support such as a sheet or web of a polymer or of paper and with conventional printing methods, and that cause as few environmental problems as possible upon manufacture, use, disposal and destruction of the device.
Another object of the present invention is to provide an electrochromic device, in which the utilised electrochromic material is in itself electrically conducting.
Another object of the present invention is to provide a combination of electrochromic systems for displays of more than one colour.
Still another object of the invention is to provide a bi-stable electrochromic display, wherein the induced colour changes remain after removal of the applied potential difference.
A further object of the invention is to provide a process for the manufacture of such an electrochromic device, which process utilises conventional printing methods or other deposition techniques that are well known, relatively unexpensive and easily scaled up.
The aforementioned and other objects are realised by the electrochromic device according to the present invention. Thus, a supported or self-supporting electrochromic device is provided, comprising:
at least one electrochromic element comprising (i) at least one material that is electrically conducting in at least one oxidation state and (ii) at least one electrochromic material, wherein said materials (i) and (ii) can be the same or different,
at least one layer of a solidified electrolyte which is in direct electrical contact with said electrochromic element, and
at least two electrodes adapted to be electrically connected to a voltage supply so as to create a difference in potential therebetween;
each of said electrodes being in direct electrical contact with at least one of said electrolyte layer(s) and not in direct electrical contact with said electrochromic element.
The electrochromic device according to the invention is particularly advantageous in that a display can be realised were the electrodes only cover a fraction of the solidified electrolyte with which they are in direct electrical contact, offering a substantial freedom when designing devices. Thus, in preferred embodiments of the invention, the electrodes cover between 0,01% and 50% of the area of the electrolyte layer(s), for example between 0,01% and 25%, or between 0,01% and 10%.
In one embodiment of the invention, an electrochromic device is provided, in which-the electrodes are arranged side by side in a plane. The electrodes then form an electrode layer, which can be deposited on a support in a conventional manner, and patterned in any desirable fashion. This is of special interest in the realisation of electrochromic displays. Also, when this arrangement of electrodes is used, the connections formed with the electrolyte are preferably made with only one layer of said electrolyte.
The invention provides an electrochromic device, where the external circuit supplying voltage to the device is not in electrical contact with the electrochromic element. The voltage applied to the electrodes induces an electric field in the electrolyte, which then surprisingly gives rise to an electrochromic colour change in the electrochromic element. This surprising possibility of addressing an electrochromic element through an electrolyte opens up many applications for the realisation of electrochromic devices. Thus, the electrochromic device according to the invention is advantageous in that it has no need for transparent electrode materials, since colour changes can take place distant from the electrodes. This offers the possibility of realising display embodiments where the electrodes are hidden on the side or on the back of the display. This feature also gives the user the freedom to use the electrodes as part of the device, for example as frames or contour lines in a display. Thus, possible embodiments of the invent,on include a completely transparent display that can be realised without the need for transparent electrodes.
In some embodiments of the invention, the electrolyte is in the form of a continuous layer to which the electrodes are applied, giving rise to a dynamic device in which application of voltage results in a colour change that is reversed upon removing the voltage. In other embodiments of the present invention, an electrochromic device is provided in which the electrolyte is patterned between electrodes The conduction of ions in this device is then interrupted, so that the application of voltage to the electrochemical cell of the device results in reduction and oxidation reactions that are not reversed upon removing the voltage. Thus, bi-stable switching between states is made possible by these accumulator-like properties of such embodiments of the device.
In embodiments of the invention, an electrochromic device is provided, which comprises at least one further electrochromic material to complement said electrochromic material in the electrochromic element. This makes it possible to realise devices with more than one colour, with for example one colour-generating oxidation reaction and one colour-generating reduction reaction taking place simultaneously at different locations in the device, As a further example, redox reactions giving rise to different colours at the same location, but at different applied voltages, can be designed. This further electrochromic material can be provided within the solidified electrolyte or within the electrochromic element, which then for example comprises an electrochromic redox pair.
Embodiments of the device of the invention may also comprise a redox active material which does not in itself give rise to electrochromic effects. Such a material may fulfil any or both of the following two roles: (i) In some arrangements of the electrochromic device according to the invention, the electrochromic material of the entire volume of the electrochromic element can not be completely oxidised or reduced in the absence of a complementary redox reaction; rather, only part of the material will be oxidised or reduced, respectively. Thus, the addition of a further redox active material makes it possible to fully oxidise or reduce the electrochromic material. (ii) The electrochromic material may be sensitive to over-oxidation, occurring at too high an applied voltage, and destroying the electrochromic material rendering it useless. A further redox active material comprised in the device may serve the function of protecting the electrochromic material from such over-oxidation, through restricting the electric polarisation in the electrochromic element to a value below a threshold value. At this threshold value, the protective, further redox active material will instead be oxidised, protecting the electrochromic material from a polarisation that would otherwise destroy it. As is readily appreciated by the skilled man in the light of what is discussed above, a suitably chosen redox active material, exhibiting electrochromic effects, could serve the function of providing a complementary, colour-generating reaction, at the same time as it provides either or both of the beneficial effects of protection against over-oxidation and enabling of complete reduction/oxidation of the first electrochromic material.
In some preferred embodiments of the invention, the electric field(s) causing the colour changes in the electrochromic element are generated in a dynamic fashion, so that displays with animated effects can be realised. Preferably, more than two individually addressed electrodes are used, and these can be positioned in a tailored manner so as to create animated elements in the display. Different and varying potentials can be applied to these electrodes, giving rise to variable electric fields in the electrolyte, by way of which the animated effects are controlled. Especially interesting is the fact that these animated effects can be realised without the need for individually addressable pixels or segments. This possibility to create dynamic effects (dynamic dedicated displays) by superposition of electric fields from several electrodes is only possible because of the fact that there is no direct electrical contact of the electrodes with the electrochromic element, but rather an ionic contact with the electrochromic element via the electrolyte, and the fact that only a fraction of the electrolyte is covered with electrodes.
Another way to generate dynamic or variable colouring effects in the electrochromic device of the invention is to use a combination of different solidified electrolytes, having different ionic conductivities. Parts of an electrochromic element, or some of a plurality of electrochromic elements, may then be in direct electrical contact with such different electrolytes. Electrochromic areas that are in contact with an electrolyte having higher ionic conductivity will colour/decolour faster than electrochromic areas that are in contact with an electrolyte having a lesser ionic conductivity, which makes possible different combinations of image elements with different colouring and decolouring speeds.
The electrochromic device according to the invention is also particularly advantageous in that it can be easily realised on a support, such as polymer film or paper. Thus, the layers of different component materials can be deposited on the support by means of conventional printing techniques such as screen printing, offset printing, ink-jet printing and flexographic printing, or coating techniques such as knife coating, doctor blade coating, extrusion coating and curtain coating, such as described in xe2x80x9cModern Coating and Drying Technologyxe2x80x9d (1992), eds E D Cohen and E B Gutoff, VCH Publishers Inc, New York, N.Y., USA. In those embodiments of the invention that utilise an electrochromic polymer (see below for materials specifications), this material can also be deposited through in situ polymerisation by methods such as electropolymerisation, UV-polymerisation, thermal polymerisation and chemical polymerisation. As an alternative to these additive techniques for patterning of the layers, it is also possible to use subtractive techniques, such as local destruction of electrochromic material through chemical or gas etching, by mechanical means such as scratching, scoring, scraping or milling, or by any other subtractive methods known in the art. An aspect of the invention provides such processes for the manufacture of an electrochromic device from the materials specified herein.
However, the invention is not limited to supported devices, as the layers of electrochromic material, electrolyte and the electrodes can be arranged in such a way that they support each other. An embodiment of the invention thus provides for a self-supporting device.
According to a preferred embodiment of the invention, the electrochromic device is encapsulated, in part or entirely, for protection of the device. The encapsulation retains any solvent needed for e g the solidified electrolyte to function, and also keeps oxygen from disturbing the electrochemical reactions in the device. Encapsulation can be achieved through liquid phase processes. Thus, a liquid phase polymer or organic monomer can be deposited on the device using methods such as spray-coating, dip-coating or any of the conventional printing techniques listed above. After deposition, the encapsulant can be hardened for example by ultraviolet or infrared irradiation, by solvent evaporation, by cooling or through the use of a two-component system, such as an epoxy glue, where the components are mixed together directly prior to deposition. Alternatively, the encapsulation is achieved through lamination of a solid film onto the electrochromic device. In preferred embodiments of the invention, in which the layers of the electrochromic device are arranged in a sheet-like configuration, the support of the device can function ads the bottom encapsulant. In this case encapsulation is made more convenient in that only the top of the sheet needs to be covered with liquid phase encapsulant or laminated with solid film.
In some embodiments of the invention, the support is itself soaked in electrolyte, so that layers of support material and electrolyte coincide. It is then possible to deposit electrically conducting, electrochromic material on one side of the support, which in this case typically is paper. On the other side, an electrode layer can be deposited, which is in direct electrical contact with the electrolyte layer soaked into the support. This electrode layer can be made sparse enough to allow for an interspersing layer of electrochromic material on this side of the support also. Thus, it is possible to easily realise a two-sided display (this aspect of the invention is further explored in relation to FIG. 5 below). Alternatively, the device according to the invention may comprise one or more electrochromic element(s) that are completely surrounded by electrolyte, which is then preferably transparent on at least one side of the element(s). The other side may be the electrolyte-soaked support mentioned above.
According to the invention, the electrodes are in direct electrical contact with the electrolyte. In cases where there are more than one layer of electrolyte, all electrodes need not be in contact with the same layer.
As discussed above, the addressing of an electrochromic element through an electrolyte opens up many possibilities for the realisation of electrochromic devices. However, this principle may be generally used in any circumstance where there is a need for addressing, or applying an electric voltage to, an electrochemically active element. The present invention thus, in a further aspect, provides a method for the application of an electric voltage to an electrochemically active element in direct electrical contact with an electrolyte, wherein electrodes of a voltage supply are brought into direct electrical contact with the electrolyte only, so as to generate an electric field in the electrolyte, which electric field in turn gives rise to a voltage induced within the electrochemically active element through its interface with the electrolyte.
The electrochemically active element in this aspect of the invention may be any element comprising an electrochemically active material, the properties of which may be altered through the application of an electric voltage. Thus, the electrochemically active element may certainly be an electrochromic element, such as in the aspects of the invention discussed above, but the method according to this aspect of the invention is equally applicable to such electrochemically active elements as transistor channels and actuators (xe2x80x9cmicromusclesxe2x80x9d). Furthermore, the method of applying a voltage via an electrolyte in the indirect fashion of this aspect of the invention offers possibilities of designing and addressing novel electrochemically active elements with a range of different functionalities.
Further objects and purposes of the present invention will be clear from the following drawings and detailed description of specific embodiments thereof. These specifications and drawings are intended as illustrations of the invention as claimed, and are not to be seen as limiting in any way,